Nowadays, end mills are most commonly made of cemented carbide due to a combination of factors, in particular the balance of roughness and toughness qualities thereof in combination with a relatively cost effective price.
The present application relates to a ceramic face mill. Ceramic has for decades been known as one of the materials which can be used for machining, but, apart from relatively small cutting inserts, is rarely used due to being comparatively more brittle and substantially more expensive than other materials such as cemented carbide.
Certain exotic materials, such as Inconel, are difficult to machine due to extreme heat generated during machining which quickly degrade a cutting tool. Consequently, these materials are normally machined at a low cutting speed (e.g. about 25 m/min). For such materials, the disadvantages of ceramic, mentioned above, are partially offset by a ceramic material's comparatively higher temperature tolerance than ceramic carbide. Such temperature quality is discussed in more detail in U.S. Pat. No. 8,647,025 entitled “monolithic ceramic end mill”.
However, regardless of the beneficial temperature quality, the ceramic end mill disclosed in U.S. Pat. No. 8,647,025 discloses significant wear. To elaborate, even though it is stated there that “The end mill was examined and found to have little chipping . . . ” (col. 5, lines 47 and 48), the present applicant notes that a skilled person would understand the amount of wear described in the examples (ranging from 0.16 mm to 0.40 mm) is not what would be considered “little chipping”. To the contrary, the values given are considerably greater than the wear normally tolerated for a similar diameter tool (in this example the diameter being 8 mm) made of cemented carbide. For example, for an 8 mm diameter endmill of cemented carbide the present applicant's internal standard for acceptable wear is 0.08 mm, which is half of the lowest wear example (0.16 mm) given. Nonetheless, the comparatively high wear is not surprising due to the known comparatively brittle nature of ceramic.
The present application is further concerned with an end mill dedicated to facing machining applications, i.e. a face mill, which primarily machines with the cutting edge at a cutting end face as opposed to along the periphery thereof.
More particularly, the present application is directed to a face mill having a circular arc profile. Even if not stated explicitly, it will be understood that all face mills subject the present application have a circular arc profile. Similarly, even if the word “ceramic” is not used, it will be understood that at least the cutting portion of the face mill, even if not explicitly stated is made of a ceramic material (or stated in other words a “ceramic cutting portion” or a “ceramic face mill”). It will be understood that these statements refer at least to a ceramic substrate, and that the cutting portion or entire face mill could have a non-ceramic coating.
A circular arc profile is presented during rotation thereof about a rotation axis and viewed in a direction perpendicular to the rotation axis. For the purposes of the specification and claims, this will be called a “profile view”. The circular arc profile defines a portion of an imaginary circle. The circle has a circle center point, axial and radial tangent lines, axial and radial tangent points, and a radius magnitude measurable from the circle center point to the circular arc profile. The axial tangent point is located at an intersection of the circle and the axial tangent line which extends forwardly from the circle center point and in a direction parallel with a rotation axis of the face mill. The radial tangent point is located at an intersection of the circle and the radial tangent line which extends radially outward from the circle center point and in a direction perpendicular with the rotation axis. For ease of understanding, a cutting edge of a tooth of a face mill can be theoretically divided into three sub-edges, namely an axial sub-edge located at a cutting end face of a face mill, a radial sub-edge located along a periphery of a cutting portion of an face mill, and a corner sub-edge extending from the axial sub-edge to the radial sub-edge. More precisely, a corner sub-edge can be defined as extending from a radial tangent point to an axial tangent point, a radial sub-edge can be defined as extending from a radial tangent point in a direction away from the corner sub-edge, and an axial sub-edge can be defined as extending from the axial tangent point in direction away from the corner sub-edge. A circular arc profile is exemplified in U.S. Pat. No. 9,517,515, the disclosure of which is incorporated herein by reference.
It will be understood that the circle and associated lines, planes, tangent points and radius magnitude, are imaginary and hence are not visible features on a face mill but rather are derivable through the construction thereof.
The present application also relates to face mills with blended gashes, blended gashes being exemplified in, for example, U.S. Pat. No. 8,858,128 of the present applicant and the citations thereof, i.e. National Aerospace Standard 986 (1973; sheet no. 55), and U.S. Pat. No. 8,414,228.
In view of the disadvantageous nature of an end mill with a ceramic cutting portion, at least when compared with cemented carbide, it is clear a unique design is needed to make such design economically feasible.